We construct a new sample of ∼1700 solar neighbourhood halo subdwarfs from the Sloan Digital Sky Survey (SDSS), selected using a reduced proper-motion diagram. Radial velocities come from the SDSS ...spectra and proper motions from the light-motion curve catalogue of Bramich et al. Using a photometric parallax relation to estimate distances gives us the full phase-space coordinates. Typical velocity errors are in the range 30–50 km s−1. This halo sample is one of the largest constructed to date and the disc contamination is at a level of ≲1 per cent. This enables us to calculate the halo velocity dispersion to excellent accuracy. We find that the velocity dispersion tensor is aligned in spherical polar coordinates and that (σr, σφ, σθ) = (143 ± 2, 82 ± 2, 77 ± 2) km s−1. The stellar halo exhibits no net rotation, although the distribution of vφ shows tentative evidence for asymmetry. The kinematics are consistent with a mildly flattened stellar density falling with distance like r−3.75. Using the full phase-space coordinates, we look for signs of kinematic substructure in the stellar halo. We find evidence for four discrete overdensities localized in angular momentum and suggest that they may be possible accretion remnants. The most prominent is the solar neighbourhood stream previously identified by Helmi et al., but the remaining three are new. One of these overdensities is potentially associated with a group of four globular clusters (NGC 5466, NGC 6934, M2 and M13) and raises the possibility that these could have been accreted as part of a much larger progenitor.
We present the first data release of the Radial Velocity Experiment (RAVE), an ambitious spectroscopic survey to measure radial velocities and stellar atmosphere parameters (temperature, metallicity, ...and surface gravity) of up to one million stars using the Six Degree Field multiobject spectrograph on the 1.2 m UK Schmidt Telescope of the Anglo-Australian Observatory. The RAVE program started in 2003, obtaining medium-resolution spectra (median R = 7500) in the Ca-triplet region (8410-8795 A) for southern hemisphere stars drawn from the Tycho-2 and SuperCOSMOS catalogs, in the magnitude range 9 < I < 12. The first data release is described in this paper and contains radial velocities for 24,748 individual stars (25,274 measurements when including reobservations). Those data were obtained on 67 nights between 2003 April 11 and 2004 April 3. The total sky coverage within this data release is ~4760 deg2. The average signal-to-noise ratio of the observed spectra is 29.5, and 80% of the radial velocities have uncertainties better than 3.4 km s-1. Combining internal errors and zero-point errors, the mode is found to be 2 km s-1. Repeat observations are used to assess the stability of our radial velocity solution, resulting in a variance of 2.8 km s-1. We demonstrate that the radial velocities derived for the first data set do not show any systematic trend with color or signal-to-noise ratio. The RAVE radial velocities are complemented in the data release with proper motions from Starnet 2.0, Tycho-2, and SuperCOSMOS, in addition to photometric data from the major optical and infrared catalogs (Tycho-2, USNO-B, DENIS, and the Two Micron All Sky Survey). The data release can be accessed via the RAVE Web site.
We use Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5) u, g, r, i, z photometry to study Milky Way halo substructure in the area around the north Galactic cap. A simple color cut (g - r < 0.4) ...reveals the tidal stream of the Sagittarius dwarf spheroidal galaxy, as well as a number of other stellar structures in the field. Two branches (A and B) of the Sagittarius stream are clearly visible in an RGB composite image created from three magnitude slices, and there is also evidence for a still more distant wrap behind the A branch. A comparison of these data with numerical models suggests that the shape of the Galactic dark halo is close to spherical.
We announce the discovery of a new dwarf galaxy, Leo T, in the Local Group. It was found as a stellar overdensity in the Sloan Digital Sky Survey Data Release 5 (SDSS DR5). The color-magnitude ...diagram of Leo T shows two well-defined features, which we interpret as a red giant branch and a sequence of young, massive stars. As judged from fits to the color-magnitude diagram, it lies at a distance of 420 kpc and has an intermediate-age stellar population with a metallicity of Fe/H = -1.6, together with a young population of blue stars of age 200 Myr. There is a compact cloud of neutral hydrogen with mass 10 super(5) M sub( )and radial velocity +35 km s super(-1) coincident with the object visible in the HIPASS channel maps. Leo T is the smallest, lowest luminosity galaxy found to date with recent star formation. It appears to be a transition object similar to, but much lower luminosity than, the Phoenix dwarf.
We present an analysis of the substructure revealed by RR Lyraes in Sloan Digital Sky Survey Stripe 82, which covers in declination on the celestial equator over the right ascension range to . We use ...the new public archive of light-motion curves in Stripe 82, published by Bramich et al. in 2008, to identify a set of high-quality RR Lyrae candidates. Period estimates are determined to high accuracy using a string-length method. A subset of 178 RR Lyraes with spectrally derived metallicities are employed to derive metallicity–period–amplitude relations, which are then used, together with archive magnitude data and light-curve Fourier decomposition, to estimate metallicities and hence distances for the entire sample. The RR Lyraes lie 5–115 kpc from the Galactic Centre, with distance estimates accurate to ∼8 per cent. The RR Lyraes are further divided into subsets of 316 RRab types and 91 RRc types based on their period, colour and metallicity. We fit a smooth density law to the distribution as a simple representation of the data. For Galactocentric radii 5–25 kpc the number density of RR Lyraes falls as r−2.4, but beyond 25 kpc, the number density falls much more steeply, as r−4.5. However, we stress that in practice the density distribution is not smooth, but dominated by clumps and substructure. Samples of 55 and 237 RR Lyraes associated with the Sagittarius Stream and the Hercules–Aquila Cloud, respectively, are identified. Hence, ∼70 per cent of the RR Lyraes in Stripe 82 belong to known substructure, and the sharp break in the density law reflects the fact that the dominant substructure in Stripe 82 – the Hercules–Aquila Cloud and the Sagittarius Stream – lie within 40 kpc. In fact, almost 60 per cent of all the RR Lyraes in Stripe 82 are associated with the Hercules–Aquila Cloud alone, which emphasizes the cloud's pre-eminence. Additionally, evidence of a new and distant substructure – the Pisces Overdensity– is found, consisting of 28 faint RR Lyraes centred on Galactic coordinates (ℓ≈ 80°, b≈−55°), with distances of ∼80 kpc. The total stellar mass in the Pisces Overdensity is ∼104 M⊙ and its metallicity is Fe/H∼−1.5.
The latest Sloan Digital Sky Survey data reveal a prominent bifurcation in the distribution of debris of the Sagittarius dwarf spheroidal (Sgr) beginning at a right ascension of a-190. Two branches ...of the stream (A and B) persist at roughly the same heliocentric distance over at least 50 of arc. There is also evidence for a more distant structure (C) well behind the A branch. This paper provides the first explanation for the bifurcation. It is caused by the projection of the young leading (A) and old trailing (B) tidal arms of the Sgr, while the old leading arm (C) lies well behind A. This explanation is only possible if the halo is close to spherical, as the angular difference between the branches is a measure of the precession of the orbital plane.
We announce the discovery of a new satellite of the Milky Way in the constellation of Bootes at a distance of 660 kpc. It was found in a systematic search for stellar overdensities in the north ...Galactic cap using Sloan Digital Sky Survey Data Release 5. The color-magnitude diagram shows a well-defined turnoff, red giant branch, and extended horizontal branch. Its absolute magnitude is M sub(v) 6 -5.8 mag, which makes it one of the faintest galaxies known. The half-light radius is 6220 pc. The isodensity contours are elongated and have an irregular shape, suggesting that Boo may be a disrupted dwarf spheroidal galaxy.
We have used the self-consistent vertical disc models of the solar neighbourhood presented in Paper I, which are based on different star formation histories (SFRs) and fit the local kinematics of ...main-sequence stars equally well, to predict star counts towards the North Galactic Pole (NGP). We combined these four different models with the local main sequence in the filter system of the Sloan Digital Sky Survey (SDSS) and predicted the star counts in the NGP field with b > 80°. All models fit the Hess diagrams in the F-K dwarf regime better than ± 20 per cent and the star number densities in the solar neighbourhood are consistent with the observed values. The χ2 analysis shows that model A is clearly preferred with systematic deviations of a few per cent only. The SFR of model A is characterized by a maximum at an age of 10 Gyr and a decline by a factor of 4 to the present-day value of 1.4 M⊙ pc−2 Gyr−1. The thick disc can be modelled very well by an old isothermal simple stellar population. The density profile can be approximated by a sech
function. We found a power-law index αt= 1.16 and a scaleheight h
t= 800 pc corresponding to a vertical velocity dispersion of σt= 45.3 km s−1. About 6 per cent of the stars in the solar neighbourhood are thick-disc stars.
In this Letter, we announce the discovery of a new dwarf satellite of the Milky Way, located in the constellation Canes Venatici. It was found as a stellar overdensity in the north Galactic cap using ...Sloan Digital Sky Survey Data Release 5 (SDSS DR5). The satellite's color-magnitude diagram shows a well-defined red giant branch as well as a horizontal branch. As judged from the tip of the red giant branch, it lies at a distance of 6220 kpc. Based on the SDSS data, we estimate an absolute magnitude of M sub(v) 6 - 7.9, a central surface brightness of k sub(0,V) 6 28 mag arcsec super(-2), and a half-light radius of 6'.5 (6550 pc at the measured distance). The outer regions of Canes Venatici appear extended and distorted. The discovery of such a faint galaxy in proximity to the Milky Way strongly suggests that more such objects remain to be found.
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